How do divers communicate over long distances? If they don't, that's something worth solving in its own right. If they do, couldn't the ROV use the same thing?
Light signals. You can say 3 things over any distance, which are "OK" (as a question or a response), "attention please" and "OMG OUT OF GAS NEED HELP IMMEDIATELY".
Given clear visibility, which gets rarer and rarer with distance - and even more so in constrained spaces, which are unlikely to be pure bedrock. Also, line of sight.
Indeed. So you see operating a ROV like that is a non-starter.
I never said otherwise; are you sure you are not replying to somebody else?